A digital image is a collection of pixels, typically arranged in an array, which defines an optically formed reproduction of one or more objects, backgrounds or other features of a scene. In a digital image, each of the pixels represents or identifies a color or other light condition associated with a portion of such objects, backgrounds or features. For example, a black-and-white digital image includes a single bit for representing a light condition of the pixel in a binary fashion (e.g., either black or white), while a grayscale digital image may represent the light condition in multiple bits (e.g., two to eight bits for defining tones of gray in terms of percentages or shares of black-and-white), and a color digital image may include groups of bits corresponding to each of a plurality of base colors (e.g., red, green or blue), and the groups of bits may collectively represent a color associated with the pixel. One common digital image is a twenty-four bit (24-bit) color digital image, in which each of the pixels includes three channels of eight bits each, including a first channel of eight bits for describing an extent of red within a pixel, a second channel of eight bits for describing an extent of green within the pixel, and a third channel of eight bits for describing an extent of blue within the pixel.
A depth image, or a depth map, is also a collection of pixels that defines an optically formed reproduction of one or more objects, backgrounds or other features of a scene. Unlike the pixels of a digital image, however, each of the pixels of a depth image represents or identifies not a light condition or color of such objects, backgrounds or features, but a distance to objects, backgrounds or features. For example, a pixel of a depth image may represent a distance between a sensor of an imaging device that captured the depth image (e.g., a depth camera or range sensor) and the respective object, background or feature to which the pixel corresponds.
Various efforts have been undertaken to incorporate or interpose depth or distance information into the pixels of a digital image. For example, where a color image includes three channels of bits each corresponding to one of the base colors of red, green and blue (viz., an RGB digital image), a fourth channel of bits corresponding to depths to objects may be appended to the three channels of bits, and a composite color and depth image (e.g., an RGBz or RGBD image) may be formed thereby. However, incorporating additional bits of depth data, e.g., additional channels of such bits, into color, grayscale or black-and-white imaging data necessarily increases the sizes of files in which such imaging data is stored, even where the color, grayscale or black-and-white imaging data is not of the highest resolution.